Here are a few of my handy tips to help you decide whether you should go ahead and buy that new X:

(1) Think about cost in terms of the work you had to do to buy it, not just the sale price. You are (most often) buying X with post-tax income so add your tax back on.

(2) Think about the opportunity cost of making the purchase, or "what can't I afford if I buy X?"

(3) Consider whether you are just buying X because it is a status object and whether there is a cheaper alternative (do you really need that merc when you can get from a->b in a honda? and so on).

In choosing what things are important to spend money on I'd generally echo others comments - people looking back over their lifetimes tend to value experiences they had more than junk they bought so spend your money buying experiences.